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Continuity Equation for multicomponent simulation 

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December 2, 2019, 14:24 
Continuity Equation for multicomponent simulation

#1 
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Lucas
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I'm working with a pair of gases (ethanol e air), one of them is adsorbed by a porous medium. The ideia to describe the phenomena is to add a mass source term to the continuity equation by creating a subdomain which englobes the whole porous medium, then in the settings I can add a source term to the continuity equation (as well as others equation like mass fraction to the ethanol that I didn't use). My question is since in the settings of Sources I can only add the source term to the continuity and it seems that I am adding the source term to the global continuity equation. Is there a way to add a source to the continuity equation of the ethanol?


December 2, 2019, 16:45 

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Careful with your mass balances.
A term in the material/multicomponent equation is to be treated carefully, it should not add mass to the system, but drive the transport of the material. Adding mass to the system it should only be done using continuity sources. A continuity source must be specified in detail, i.e you must include its composition. If you attempt to do it by adding sources to the material components you must be extremely careful to conserve global mass. Recall the summation of all the material component equations should be the same as the global continuity equation. That is the reason there is a constrain material component; otherwise, the system will over specified. 

December 3, 2019, 09:57 

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It is a print from CFX, the continuity that is chosen is the correspondent to the ethanol ou the global one? In the fluid models settings I put ethanol to have a transport equation and air to be constraint. I am reading the documentation and doing some tests to find out. Any news I will share here. 

December 3, 2019, 11:49 

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If Sm (mass source strength)
 is positive only ethanol is being added  and, if negative only ethanol is being removed (here the sink options kicks in for the specific behavior) The corresponding source terms are included in all solved transport equations. It should work for you. 

December 5, 2019, 01:35 

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December 7, 2019, 05:53 

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Thanks for the reply, but, apparently, it didn't work. Since the mass fraction increased, instead of decrease. Antanas and Opaque, is there another way of making CFX understand that the source term is only to ethanol?


December 8, 2019, 18:54 

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Could you post the settings for your source?


December 9, 2019, 10:25 

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Sure, Opaque. I am attaching a photo and the whole code.
LIBRARY: CEL: EXPRESSIONS: A = 132.89 [s^1] E = 139.5 [kJ mol^1] Ea = 22.97 [kJ mol^1] Ps = 0.1041 [Pa] R0 = 0.0083145 [kJ mol^1 K^1] Sm = (1volpor)*roP*k1*(qeq) fm = 1fluid.air.mf k1 = A*e^((Ea/(R0*T))) q = qe*(1e^(k1*t)) qe = qs*exp((((R0*(T/E))*ln(Ps/(ptot))))^2) qs = 1.2 [kg kg^1] roP = 464.1 [kg m^3] END END MATERIAL: ADSOR Material Group = User Option = Pure Substance Thermodynamic State = Solid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 464.1 [kg m^3] Molar Mass = 12 [g mol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 1375 [J kg^1 K^1] END REFERENCE STATE: Option = Automatic END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 0.2 [W m^1 K^1] END END END MATERIAL: Air Ideal Gas Material Description = Air Ideal Gas (constant Cp) Material Group = Air Data,Calorically Perfect Ideal Gases Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Molar Mass = 28.96 [kg kmol^1] Option = Ideal Gas END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 1.0044E+03 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Specific Enthalpy = 0. [J/kg] Reference Specific Entropy = 0. [J/kg/K] Reference Temperature = 25 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 1.831E05 [kg m^1 s^1] Option = Value END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 2.61E2 [W m^1 K^1] END ABSORPTION COEFFICIENT: Absorption Coefficient = 0.01 [m^1] Option = Value END SCATTERING COEFFICIENT: Option = Value Scattering Coefficient = 0.0 [m^1] END REFRACTIVE INDEX: Option = Value Refractive Index = 1.0 [m m^1] END END END MATERIAL: Mix Material Group = User Materials List = air,ethanol Option = Variable Composition Mixture Thermodynamic State = Gas MIXTURE PROPERTIES: Option = Ideal Mixture EQUATION OF STATE: Option = Ideal Mixture END SPECIFIC HEAT CAPACITY: Option = Ideal Mixture END END END MATERIAL: N2 Material Group = User Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Density = 0.8556 [kg m^3] Molar Mass = 28.0134 [g mol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 29.271 [cal g^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Temperature = 399 [K] END END END MATERIAL: air Material Group = User Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Density = 1.185 [kg m^3] Molar Mass = 28.96 [kg kmol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 1.0044E+03 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Specific Enthalpy = 0 [J kg^1] Reference Specific Entropy = 0 [J kg^1 K^1] Reference Temperature = 25 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 20.86e6 [Pa s] Option = Value END THERMAL EXPANSIVITY: Option = Value Thermal Expansivity = 0.003356 [K^1] END END END MATERIAL: ethanol Material Group = User Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Density = 736.1 [kg m^3] Molar Mass = 46.0684 [g mol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 3.19 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Temperature = 25 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 10.4e6 [Pa s] Option = Value END END END MATERIAL: iPentano Material Group = User Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Density = 2.2337 [kg m^3] Molar Mass = 72.15 [g mol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 154.15 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Temperature = 400 [K] END END END MATERIAL: nPentano Material Group = User Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Density = 2.2345 [kg m^3] Molar Mass = 72.17 [g mol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 2.13 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Temperature = 400 [K] END END END END FLOW: Flow Analysis 1 SOLUTION UNITS: Angle Units = [rad] Length Units = [m] Mass Units = [kg] Solid Angle Units = [sr] Temperature Units = [K] Time Units = [s] END ANALYSIS TYPE: Option = Transient EXTERNAL SOLVER COUPLING: Option = None END INITIAL TIME: Option = Automatic with Value Time = 0 [min] END TIME DURATION: Option = Total Time Total Time = 20 [min] END TIME STEPS: Option = Timesteps Timesteps = 1 [min] END END DOMAIN: Default Domain Coord Frame = Coord 0 Domain Type = Porous Location = ADSORVENT BOUNDARY: In Boundary Type = INLET Location = IN BOUNDARY CONDITIONS: COMPONENT: ethanol Mass Fraction = 0.4 Option = Mass Fraction END FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Normal Speed = 0.03 [m s^1] Option = Normal Speed END TURBULENCE: Option = Medium Intensity and Eddy Viscosity Ratio END END END BOUNDARY: Out Boundary Type = OPENING Location = OUT BOUNDARY CONDITIONS: COMPONENT: ethanol Mass Fraction = fm Option = Mass Fraction END FLOW DIRECTION: Option = Normal to Boundary Condition END FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Opening Pressure and Direction Relative Pressure = 1 [atm] END TURBULENCE: Option = Medium Intensity and Eddy Viscosity Ratio END END END BOUNDARY: Wall Boundary Type = WALL Location = WALL BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END DOMAIN MODELS: BUOYANCY MODEL: Option = Non Buoyant END DOMAIN MOTION: Option = Stationary END MESH DEFORMATION: Option = None END REFERENCE PRESSURE: Reference Pressure = 1 [atm] END END FLUID DEFINITION: fluid Material = Mix Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID MODELS: COMBUSTION MODEL: Option = None END COMPONENT: air Option = Constraint END COMPONENT: ethanol Option = Transport Equation END HEAT TRANSFER MODEL: Fluid Temperature = 352 [K] Option = Isothermal END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = k epsilon END TURBULENT WALL FUNCTIONS: Option = Scalable END END POROSITY MODELS: AREA POROSITY: Option = Isotropic END LOSS MODEL: Loss Velocity Type = Superficial Option = Isotropic Loss ISOTROPIC LOSS MODEL: Option = Permeability and Loss Coefficient Permeability = 8.379e12 [m^2] Resistance Loss Coefficient = 3.447e5 [m^1] END END VOLUME POROSITY: Option = Value Volume Porosity = 0.4075 END END SOLID DEFINITION: adsor Material = ADSOR Option = Material Library MORPHOLOGY: Option = Continuous Solid END END SOLID MODELS: HEAT TRANSFER MODEL: Option = Isothermal Solid Temperature = 352 [K] END THERMAL RADIATION MODEL: Option = None END END SUBDOMAIN: TermoFonte Coord Frame = Coord 0 Location = ADSORVENT SOURCES: EQUATION SOURCE: continuity Multiply by Porosity = Off Option = Fluid Mass Source Sink Option = Local Mass Fractions and Temperature Source = Sm VARIABLE: ed Option = Value Value = 1 [m^2 s^3] END VARIABLE: ethanol.mf Option = Value Value = 1 [] END VARIABLE: ke Option = Value Value = 1 [m^2 s^2] END VARIABLE: vel Option = Cartesian Vector Components xValue = 1 [m s^1] yValue = 1 [m s^1] zValue = 1 [m s^1] END END END END END INITIALISATION: Option = Automatic INITIAL CONDITIONS: Velocity Type = Cartesian CARTESIAN VELOCITY COMPONENTS: Option = Automatic with Value U = 0 [m s^1] V = 2.52e3 [m s^1] W = 0 [m s^1] END COMPONENT: ethanol Mass Fraction = 0.3 Option = Automatic with Value END STATIC PRESSURE: Option = Automatic with Value Relative Pressure = 0 [Pa] END TURBULENCE INITIAL CONDITIONS: Option = Medium Intensity and Eddy Viscosity Ratio END END END OUTPUT CONTROL: RESULTS: File Compression Level = Default Include Mesh = No Option = Selected Variables Output Variables List = Absolute Pressure,Pressure,ethanol.Molar \ Concentration,ethanol.Mass Fraction,ethanol.Mass Concentration END TRANSIENT RESULTS: Transient Results 1 File Compression Level = Default Include Mesh = On Option = Selected Variables Output Boundary Flows = All Output Equation Residuals = All Output Variable Operators = All Output Variables List = ethanol.Molar Concentration,ethanol.Mass \ Fraction,ethanol.Mass Concentration,ethanol.Molar Fraction OUTPUT FREQUENCY: Option = Every Timestep END END END SOLVER CONTROL: Turbulence Numerics = First Order ADVECTION SCHEME: Option = High Resolution END CONVERGENCE CONTROL: Maximum Number of Coefficient Loops = 10 Minimum Number of Coefficient Loops = 1 Timescale Control = Coefficient Loops END CONVERGENCE CRITERIA: Residual Target = 1.E4 Residual Type = RMS END TRANSIENT SCHEME: Option = Second Order Backward Euler TIMESTEP INITIALISATION: Option = Automatic END END END END COMMAND FILE: Version = 18.2 Results Version = 18.2 END SIMULATION CONTROL: EXECUTION CONTROL: EXECUTABLE SELECTION: Double Precision = No Large Problem = No END INTERPOLATOR STEP CONTROL: Runtime Priority = Standard MEMORY CONTROL: Memory Allocation Factor = 1.0 END END PARALLEL HOST LIBRARY: HOST DEFINITION: lordluan Host Architecture String = winntamd64 Installation Root = C:\Program Files\ANSYS Inc\v%v\CFX END END PARTITIONER STEP CONTROL: Multidomain Option = Automatic Runtime Priority = Standard MEMORY CONTROL: Memory Allocation Factor = 1.0 END PARTITION SMOOTHING: Maximum Partition Smoothing Sweeps = 100 Option = Smooth END PARTITIONING TYPE: MeTiS Type = kway Option = MeTiS Partition Size Rule = Automatic END END RUN DEFINITION: Run Mode = Full Solver Input File = C:\Users\Lordluan \luan \Downloads\TCC\Teste2\9_v5_transient.def Solver Results File = C:\Users\Lordluan \ luan\Downloads\TCC\Teste2\9_v5_transient_004.res END SOLVER STEP CONTROL: Runtime Priority = Standard MEMORY CONTROL: Memory Allocation Factor = 1.0 END PARALLEL ENVIRONMENT: Number of Processes = 1 Start Method = Serial END END END END 

December 9, 2019, 10:43 

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Based on your settings, the total amount of mass removed (assuming Sm < 0) would be something along
volumeInt(Sm * Ethanol.mf)@TermoFonte Your source specification is based on Local Mass Fractions, that is, it will remove whatever is "available" of Ethanol in the subdomain. If you look at the output file summary (end of the run), how much is the mass flow for the subdomain? Look at the PMass summary as well as the Ethanol.Mass Fraction summary 

December 9, 2019, 21:35 

#10  
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December 11, 2019, 00:10 

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PS Refer to SteamJet tutorial for reference 

December 11, 2019, 11:39 

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Let me explain the issue with sinks better.
Let say at a given control volume there is 1 [kg] of mixture spread equally between two components, i.e. Mass Fraction = 0.5 for both. We introduce a sink, and establish a mass flow of 0.75 [kg s^1] with a composition of 0 and 1. That means, in one timestep (true or false) we are requesting a total mass for a given component that is not available in the control volume; therefore, violating the physics. Summary: sinks are trickier to implement than injections, and the requested net value must be physically possible based on the integration parameters. That is no different than an outlet with a mass flow larger than what the system can handle based on the inlet conditions and wall settings. 

December 11, 2019, 18:46 

#13 
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Opaque  but doesn't that mean that the mass sink is not matched to a mass flux entering the control volume  in other words, the time step is not converged. It will then do iterations to match the mass flux through the control volume faces with the mass leaving via the mass sink.
Doesn't that mean you can specify a mass sink as big as you like? Of course providing the system can supply the flow rate required, and providing you achieve a converged solution. I can see how larger mass sinks would be harder to converge.
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December 12, 2019, 10:48 

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The algorithm will iterate until the solution is converged and achieve the required sink if "possible", i.e. the "sink" is physically limited by boundary conditions and time step.
For a given time step is not guaranteed the physics can supply the mass flow rate being removed at a given location. A similar issue will be with an energy sink, if we remove too much energy that cannot arrive at the location either by advection or diffusion, then the temperature could become unphysical. Boundary conditions/sources/sink are limited by physics compatibility. 

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